Copper pillar is a chip-to-package interconnect technology that is entering the market. The advantages in copper lie in the extendability to finer pitch and the superior electromigration (EM) performance compared to conventional solder C4's. The copper pillar provides the enhanced EM performance but increases the die stress for chip-to-chip and chip-to-wafer joints (i.e., 3D applications). The finer pitch is due to its vertical sidewalls.
In copper pillar technology, a small amount of solder is still required to connect the copper pillars on the chips to the pad on the substrate. However, copper pillar-based solder connectors are relatively expensive to fabricate and also difficult to prevent the solder from wetting to the copper pillar sidewall. Such wetting to the sidewall will reduce the standoff, therefore limiting underfill flow and can sometimes lead to solder bridging. Thus, it is desirable to have a pillar structure based solder ball connector that is less expensive to manufacture and avoids side wall wetting.
More specifically, FIGS. 1a and 1b show a conventional copper pillar structure with wettable sidewalls. As shown in these views, the solder will flow onto the sidewalls of the copper pillars. This can cause bridging between adjacent copper pillars which, in turn, results in shorting (see, FIG. 1b). Also, although the copper pillars can be used in finer pitch applications, the solder wetting requires the copper pillars to be placed farther apart than desired. For this reason, it is not possible to provide a denser structure of connections.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.